1. Technical Field
The present disclosure relates to a transmission device and a distortion compensation method that compensate signal distortion.
2. Description of the Related Art
A class-D amplifier, equivalently, has a configuration in which output is performed with a switch between a power supply and an output, and a switch between ground and the output being alternately turned on. In an ideal operating state, power efficiency is high because there is no unnecessary through-current flowing from the power supply to ground.
However, in an actual circuit, if the switches are turned on at the same time, through-current flows between the power supply and ground and power efficiency decreases due to signal dullness or switch timing deviation or the like. As a countermeasure for this through-current, there is a technique in which non-overlapping clocks are used. Non-overlapping clocks refer to, in the case where a switch between a power supply and an output, and a switch between ground and the output are to be alternately turned on, the provision of time domains (non-overlapping periods) in which both switches are intentionally turned off. This is a technique that prevents the generation of through-current which decreases power efficiency, is extremely effective for increasing the efficiency of a class-D amplifier, and is widely used.
In order to realize high efficiency with a power amplifier configured using a complementary metal-oxide semiconductor integrated circuit (CMOS IC), a switched-capacitor power amplifier (SCPA), which is a type of class-D amplifier, is used. An SCPA carries out amplitude modulation by changing the number of amplifiers that switch between a power supply voltage and ground.
AM-AM characteristics and AM-PM characteristics are used as indexes for evaluating the performance of a power amplifier. FIG. 1A depicts the AM-AM characteristics of a general power amplifier, and FIG. 1B depicts the AM-PM characteristics of a general power amplifier. In FIGS. 1A and 1B, the solid line represents the ideal characteristics, and the line connecting the points plotted using white squares represents the actual characteristics. From FIG. 1A and FIG. 1B, it is apparent that the way in which a signal distorts is different depending on the size of the input. Furthermore, the distortion characteristics themselves also change due to the individual variations of the power amplifiers, temperature fluctuations, and changes that occur with aging.
Adaptive digital predistortion (hereafter, referred to as “adaptive DPD”) is known as a technique for compensating this distortion. In adaptive DPD, the inverse characteristics of the distortion of a power amplifier corresponding to the size of the input of transmission data are held in a lookup table (LUT) as correction coefficients. The LUT is appropriately updated with the output of the power amplifier serving as a feedback signal, and the correction coefficients of the updated LUT are used to perform distortion compensation. Thus, it is possible to also respond to changes in distortion characteristics caused by individual variations, temperature fluctuations, and changes that occur with aging.
A technique for carrying out distortion compensation in an IQ quadrature transmission device in which an SCPA is used is disclosed in the specification of US Unexamined Patent Application Publication No. 2012/0269293. The specification of US Unexamined Patent Application Publication No. 2012/0269293 discloses a method in which correction coefficients are obtained in accordance with the size of I components and Q components in a training mode, a two-dimensional matrix LUT is generated, and linear interpolation is performed from the correction coefficients of the generated LUT for distortion compensation to be carried out. The method is implemented in this manner because the way in which a signal distorts is different according to the size of the I component and the size of the Q component.
Furthermore, generally, in transmission devices, there is a high demand for operation at high efficiency in addition to the suppression of power leakage to adjacent channels. In response to this demand, operation at high efficiency is possible in an SCPA by switching non-overlapping (hereafter referred to as “NOL”) adjustment values in accordance with code size. The NOL adjustment values correspond to duty cycles. FIG. 2A depicts AM-AM characteristics at various NOL adjustment values (NOL=0 to 7), and FIG. 2B depicts AM-PM characteristics at various NOL adjustment values (NOL=0 to 7). Note that NOL=0 to 7 represent different duty cycles.
The AM-AM characteristics and the AM-PM characteristics when the NOL adjustment value is switched in accordance with code size are depicted in FIG. 3A and FIG. 3B, respectively. Here, switching is performed such that NOL=7 in codes 0 to 31, NOL=3 in codes 32 to 39, NOL=2 in codes 40 to 47, NOL=1 in codes 48 to 55, and NOL=0 in codes 56 to 63. From FIG. 3A and FIG. 3B, it is apparent that although gain linearity is maintained, phase fluctuations occur.